RNA interference (RNAi) is a phenomenon by which RNA molecules inhibit gene expression. RNAi occurs when a mRNA, typically transcribed from a transgene, is converted into double-stranded RNA (dsRNA) by an RNA-dependent RNA polymerase (RdRP). The dsRNA becomes a substrate for Dicer-like proteins, which cut the dsRNA into small interfering dsRNAs (siRNA) about 25 nucleotides or shorter in length. The siRNA becomes incorporated into an RNA-induced silencing complex (RISC), which targets and degrades additional transgene mRNA in a sequence-specific manner based on the sequence of the particular siRNA. The result is reduced expression of the transgene, as well as endogenous genes having sufficiently similar nucleotide sequences to that transgene. The phenomenon was first described by Fire and Mello in Caenorhabditis elegans (Fire, A. et al. (1998). Nature 391:806-11).
RNAi occurs in a wide variety of eukaryotic cells, and is known by a number of different names, including co-suppression, post transcriptional gene silencing (PTGS), and quelling. Quelling is the term usually reserved for RNAi in fungal molecular biology. Some fungi can have a strong quelling response, while other have a weak response, or no quelling response at all. Quelling was first described in Neurospora in the early 1990s (Romano, N. et al. (1992) Mol. Microbiol. 6:3343-53).